time: A Timer must be properly created before use (documentation).

Fixes golang#8776.

LGTM=bradfitz
R=rsc, bradfitz
CC=golang-codereviews
https://golang.org/cl/145710043

src/time/sleep.go

@@ -43,6 +43,7 @@ func stopTimer(*runtimeTimer) bool
 // The Timer type represents a single event.
 // When the Timer expires, the current time will be sent on C,
 // unless the Timer was created by AfterFunc.
+// A Timer must be created with NewTimer or AfterFunc.
 type Timer struct {
 	C <-chan Time
 	r runtimeTimer